This invention concerns endoprosthetic bone joint devices and more particularly such devices including a component of overall cup form to replace the acetabulum in the hip joint.
Present day acetabular cup components represent the result of several decades of clinical experience and a yet longer period of trial and development, but the use of such components is nevertheless not without difficulties.
A general difficulty is that an optimum result requires accuracy in the positioning of the component in the pelvis, but such positioning is hindered by the fact that the component has a preset overall geometry while the pelvis will vary in shape between individual patients and so give no absolute spacial frame of reference, and current securement techniques employing fast-curing acrylic bone cement give little scope for adjustment after the component has been first located. The component is usually introduced with a single smooth action to ensure that cement is evenly distributed between it and the bone, and the component is thereafter held steadily, without angular or lateral movement, while the cement is curing in order to avoid the creation of voids or other weaknesses in the securement. In practice therefore the initial positioning of the component is definitive and the attainment of an optimum result accordingly requires skill and care, while the later consequences of failure include undue component wear and/or dislocation.
Even if skill and care are exercised, there is still a difficulty in that optimum positioning of the outer and inner surfaces of the component are respectively determined by the requirements of securement and articulation but, because these requirements can varyingly differ from one patient to another, attainment of both optima is often compromised by a component of preset overall geometry.
An object of the present invention is to reduce these difficulties and to this end there is provided an endoprosthetic acetabular component of overall cup form comprising an outer part and a plurality of inner parts individually selectively co-operable with said outer part, the outer part being adapted for securement to bone, and each inner part being interconnectable with said outer part in like manner and defining a dished articulation surface having a respectively different predetermined position relative to said outer part when connected therewith.
Preferably the inner parts afford a range of articulation surface positions varying in depth and inclination relative to the outer part.
Also, it is preferred that the outer and inner parts are rotatably adjustably interconnectable relative to the mouth of the overall cup form.
The benefit of the proposed device relative to the above-mentioned difficulties is that the outer part can be secured to the bone separately with a fuller attention being paid to the requirements of securement in terms of the geometry and mechanical condition of the bone, and thereafter an appropriate one of the inner parts can be selectively interconnected to locate its dished surface in the position which is judged to be best for the purposes of articulation.
The outer part can take various forms to allow for securement to bone in accordance with established or future techniques. For example, the part can have an outermost surface with a ribbed-and-grooved or other relieved configuration for cooperation with bone cement, or the part can be apertured for the passage of screws therethrough. Indeed the outer part can itself be made available in such different forms interconnectable with the same range of inner parts in like manner to allow a choice of securement technique best suited to a bone when exposed during surgery.
Variation is also possible in the adaptation for interconnection between the outer and inner parts. It is presently preferred that such adaptation involve mutually complementary indented and detented formations on the outer and inner parts. In initially conceived forms of the invention such formations were applied to the interior and exterior surfaces respectively of the outer and inner parts. These initial forms typically involve prismatic surface forms of polygonal or other non-circular cross-sectional shape. However it is now thought advantageous to apply the complementary formations to rim surfaces of the outer and inner parts in order to facilitate associated machining or other fabrication operations. More recently conceived forms of the invention in fact typically involve castellated rim surface formations.